WASHINGTON
— In a break with half a century of bipartisan support for trade liberalization, the House heads into a deciding vote on the Central American Free Trade Agreement with nearly all Democrats opposed.

At least marginally, CAFTA is a better deal for US workers than trade agreements before it. The pact, forged in early 2004, lowers trade barriers with Costa Rica, El Salvador, Guatemala, Honduras, and Nicaragua, and includes a related agreement with the Dominican Republic. Since most exports from the region already enter US markets freely, the main change is to open Central American markets to US goods.

In a bid for Democratic votes, the Bush White House negotiated side agreements on labor and the environment, including a promise to Senate Democrats last month to spend $40 million a year to boost labor rights in the region. Last month, the Senate passed CAFTA by a 54-45 vote with 10 Democrats in support. But on the eve of this week's expected House vote, only six Democrats openly backed the agreement, although aides say that could rise to 15.

"I need to show that there are some Democrats who are willing to support Central America's last shot to join the first tier of nations. If we wait, they will be so marginalized by the growth of China and India that it will be too late," says Rep. James Moran (D) of Virginia, who is leading efforts to recruit more Democratic votes.

For supporters, the challenge goes beyond economics to partisan politics. Democrats who have been longtime supporters of free trade say they have not been consulted on key trade issues, and are unwilling to support the majority now in a tough vote.

"It's basically about polarization more than trade," says I.M. Dextler, a professor at the University of Maryland School of Public Policy and author of American Trade Politics. "With other free trade agreements, you still had more than a third of Democrats voting in favor, but CAFTA is the one that labor is making the litmus test. Combined with very poor relations between parties across the board, it makes it a tough vote."

For lawmakers in districts wracked by job losses overseas, trade liberalization is a tough sell. Democratic leaders say pacts have contributed to the loss of millions of manufacturing jobs, and that CAFTA will hurt women and minority textile workers.

"Democrats are much more sensitive to the concerns of organized labor now in a political environment when they have few staunch allies and are losing elections," says Norman Ornstein of the American Enterprise Institute.

In response, the White House and business groups shifted into high gear in a bid to find the votes to salvage the 2004 agreement. On the line: perks and promises ranging from targeted protection for specific industries to new highway projects.

On Tuesday, three Republicans said they'll support CAFTA, after the White House and Central American ministers promised to close loopholes for Chinese components that might find greater access to US markets through CAFTA. Critics say such promises are rarely enforced and don't level the playing field for US workers. "The only commitments that are binding in terms of international law are those contained in the text of the treaty itself. Side letters are meaningless," says Alan Tonelson of the US Business and Industry Council.